As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas- and light-barrier material, e.g. an aluminium foil or an ethyl vinyl alcohol (EVOH) foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating. The web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which may be fed to a folding unit to form the finished packages.
More specifically, the pillow packs substantially comprise a main portion, and opposite top and bottom end portions tapering from the main portions towards respective top and bottom sealing bands which extend substantially orthogonal to the axis of the pack. In detail, each end portion is defined by a pair of respective trapezoidal walls which extend between main portion of the pack and the relative sealing band.
Each pillow pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin projecting from respective sealing bands; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by respective trapezoidal walls.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing band.
Packaging machines for producing packages of the above type are known, substantially comprising:                an in-feed conveyor;        a folding unit receiving the pillow packs from the in-feed conveyor and adapted to fold these pillow packs to form the parallelepiped-shaped packages;        a transfer unit for transferring and up-ending the folded packages, which is arranged downstream from the folding unit and receives the sealed packages from the folding unit; and        an out-feed conveyor which receives folded packages from the transfer unit and moves them away from the packaging machine.        
Folding units are known, for example from EP-B-0887261 in the name of the same Applicant, which typically comprise:                a chain conveyor for feeding packs continuously along a forming path from a supply station to an output station;        a number of folding devices arranged in fixed positions relative to the forming path and cooperating with packs to perform relative folding operations thereon;        a heat-sealing device acting on respective triangular flaps of each pack to be folded, to melt the external layer of the packaging material and seal flaps onto respective walls of the pack; and        a pressing device cooperating with each pack to hold the triangular portions on respective walls as these portions cool.        
In detail, the chain conveyor comprises a top straight branch, a bottom straight branch and two curved portions which are opposite to each other and connect, on respective opposite sides, the top and bottom branches.
More precisely, the axes of the packs are slightly backwards inclined relative to a vertical direction when they are fed to the chain conveyor at the supply station, and are substantially vertical when packs are fed along the top branch. Furthermore, the folded packages are slightly forwards inclined relative to the vertical direction, when they reach the output station.
In other words, when moving along the forming path, the packs and the corresponding folded packages are arranged above and are, therefore, continuously supported by the chain conveyor.
The pressing device comprises three endless belts which define, between them and together with the top branch of the chain, a forming passage having a constant rectangular section, and defining the outer contour of the finished packages.
Transfer units are known, for example from EP-B-0887268 in the name of the same Applicant.
In detail, the known transfer units move the packages successively along a path from an in-feed station to an out-feed station, and simultaneously up-end the packages from an in-feed position, in which the packages are positioned with their axis tilted roughly 15° to the horizontal, into an out-feed position, in which the packages are positioned with their axis substantially vertical.
More specifically, the in-feed position of the transfer unit substantially coincides with the output station of the folding unit.
Known transfer units substantially comprise a rotary member having a number of push arms which cooperate with respective packages to push them along the path; and a fixed guide which extends along this path and cooperates with the packages to ease them from the tilted in-feed position to the out-feed position.
Though efficient, packaging machines of the above type leave room for improvement.
As a matter of fact, a wide range of modified package shapes has been developed which are different from the parallelepiped package.
In particular, packages with a slightly rounded or an octagonal cross section have been developed.
For these packages, the Applicant has found that the forming operation may require some adjustments. This is mainly due to the fact that the forming passage must be, in this case, polygonal whereas the endless belts have substantially flat surfaces cooperating with the folded package.
Furthermore, the Applicant has found that these modified packages tend to rotate about their own axis, as they are fed from the in-feed to the out-feed position.
As a result, there is some risk that turned packages stop along the path defined by the transfer unit and causes the stop of the transfer unit and, therefore, of the whole packaging machine.